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Experimental Investigation of Geologically Produced Antineutrinos with KamLAND Stanford University Department of Physics Kazumi Ishii

Outline • Geologically Produced Antineutrinos (Geoneutrinos) • KamLAND • Background Events • Results

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Structure of the Earth • Seismic data splits Earth into 5 basic regions: core, mantle, oceanic crust, continental crust, and sediment. • All these regions are solid except the outer core. Image by: Colin Rose and Dorling Kindersley

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Convection in the Earth

Image: http://www.dstu.univ-montp2.fr/PERSO/bokelmann/convection.gif

• The mantle convects even though it is solid. • It is responsible for the plate tectonics and earthquakes. • Oceanic crust is being renewed at mid-ocean ridges and recycled at trenches. 8/9/2005

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Total Heat Flow from the Earth Bore-hole Measurements

• Conductive heat flow measured from bore-hole temperature gradient and conductivity • Deepest bore-hole (12km) is only ~1/500 of the Earth’s radius. • Total heat flow 44.21.0TW (87mW/m2), or 311TW (61mW/m2) according to more recent evaluation of same data despite the small quoted errors. 5

Image: Pollack et. al

Radiogenic Heat •

238U, 232Th

and K generate 8TW, 8TW, and 3TW of radiogenic heat in the Earth

• Beta decays produce electron antineutrinos

Urey Ratio and Mantle Convection Models • Urey ratio indicates what fraction of heat dissipated comes from radiogenic heat. Urey ratio can be defined as mantle heat production Urey Ratio  mantle heat dissipation

• Some mantle convection models predict Urey ratio > ~0.7. 8/9/2005

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Discrepancy? • The measured total heat flow, 44 or 31TW, and the estimated radiogenic heat produced in the mantle, 13TW, gives Urey Ratio ~0.3 or ~0.5. • Problem with – Mantle convection model? – Total heat flow measured? – Estimated amount of radiogenic heat production rate?

• Geoneutrino can serve as a cross-check of the radiogenic heat production. 8/9/2005

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Geoneutrino Signal

• KamLAND is only sensitive to antineutrinos above 1800keV • Geoneutrinos from K decay cannot be detected with KamLAND. 8/9/2005

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U and Th in the Earth Chondritic Meteorites • U and Th concentrations in the Earth are based on measurement of chondritic meteorites. • Chondritic meteorites consist of elements similar to those in the solar photosphere. • Th/U ratio is 3.9 • Th/U ratio is known better than the absolute concentrations. 8/9/2005

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U and Th Distributions in the Earth • U and Th are thought to be absent from the core and present in the mantle and crust. – The core is mainly Fe-Ni alloy. – U and Th are lithophile (rock-loving), and not siderophile (metal-loving) elements.

• U and Th concentrations are the highest in the continental crust and continental sediment. – Mantle crystallized outward from the core-mantle boundary. – U and Th prefer to enter a melt phase. 8/9/2005

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Reference Earth Model Concentrations of U and Th • Total amounts of U and Th in the Earth are estimated from the condritic

meteorites. • Concentrations in the sediments and crusts are based on the samples on the surface, seismic data, and tectonic model. • Concentrations in the mantle are estimated by subtracting the amounts in the sediments and the crusts. Sediment Continental Crust

Oceanic Crust Mantle Core

Continental Oceanic Upper Middle Lower

U [ppm] 2.8 1.68 2.8 1.6 0.2 0.1 0.012 0

Th [ppm] 10.7 6.91 10.7 6.1 1.2 0.22 0.048 0

Geological Uncertainty • We assigned 10% for the observable geological uncertainty. • This does not include uncertainties in the total amounts or distributions of U and Th. U concentrations

U and Th concentration variations due to various crustal types contribute ~7% error in the total flux.

Variations in local U and Th concentrations contribute ~3% error in the total flux.

Neutrino Oscillations • The weak interaction neutrino eigenstates may be expressed as superpositions of definite mass eigenstates 3

 l   U li  i i 1

• The electron neutrino survival probability can be estimated as a two flavor oscillations:  1.27m122 [eV2 ]L[m]  P  E , L   1  sin 212 sin   E [MeV]    2

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KamLAND Neutrino Oscillation Measurement

• KamLAND saw an antineutrino disappearance and a spectral distortion. • KamLAND result combined with solar experiments precisely measured the oscillation parameters. 2 0.6 5 2 2 m  7.9  10 eV sin 212  0.82  0.07 12 0.5 8/9/2005

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The Expected Geoneutrino Flux • Given an Earth model and neutrino oscillation parameters, the antineutrino flux per unit energy at KamLAND is given by



d E ,r dE

  A dn E  

dE



V

3

d r'





a r '  r ' P E ,| r  r ' | 4 r  r '

2

• The decay rate per unit mass • The number of antineutrinos per decay chain per unit energy • The mass concentration as a function of position in the Earth

• The density as a function of position in the Earth • A survival probability due to neutrino oscillations, r r P E ,| r  r ' | 1 12 sin 2 212  0.59 for geoneutrino energy range. 8/9/2005

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Reference Earth Model Flux

• Expected geoneutrino flux at KamLAND – – 8/9/2005

238U

geoneutrinos: 2.34106 cm-2s-1 232Th geoneutrinos: 1.98 106 cm-2s-1 17

Expected Geoneutrino Detection Rate

• By multiplying the expected geoneutrino flux and cross-sections, detection rates for geoneutrinos from U and Th at KamLAND are – –

238U

geoneutrinos: 3.010-31 per target proton year 232Th geoneutrinos: 0.8510-31 per target proton year

Geoneutrino Map of the Earth Simulated origins of geoneutrinos detectable with KamLAND using the reference Earth model KamLAND

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Geoneutrino References • G. Marx, Menyhard N, Mitteilungen der Sternwarte, Budapest No. 48 (1960) • M.A. Markov, Neutrino, Ed. "Nauka", Moscow, 1964 • G. Eders, Nucl. Phys., 78 (1966) 657 • G. Marx, Czech. J. of Physics B, 19 (1969) 1471 • G. Marx and I. Lux, Acta Phys. Acad. Hung., 28 (1970) 63 • C. Avilez et al., Phys. Rev. D23 (1981) 1116 • L. Krauss et al., Nature 310 (1984) 191 • J.S. Kargel and J.S. Lewis, Icarus 105 (1993) 1 • R.S. Raghavan et al., Phys. Rev. Lett. 80 (1998) 635 • C.G. Rothschild, M.C. Chen, F.P. Calaprice, Geophys. Rev. Lett. 25 (1998) 1083 • F. Montovani et al., Phys. Rev. D69 (2004) 013001 8/9/2005

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Have Geoneutrinos Been Measured before?

Fred Reines’ neutrino detector (circa 1953)

By Gamow in 1953

Were Fred Reines Background Events from Geoneutrinos?

~30TW

Outline • • • •

Geoneutrinos KamLAND Background Events Results

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1km Overburden

KamLAND Detector Electronics Hut Steel Sphere, 8.5m radius

Inner detector 1325 17” PMT’s 554 20” PMT’s 34% coverage

1 kton liquidscintillator

Transparent balloon, 6.5m radius Buffer oil

Water Cherenkov outer detector 225 20” PMT’s

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Inside the Detector

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Determining Event Vertices

• Vertex determined using the photon arrival times at PMTs. • Calibrated using sources deployed down the center of the detector. 8/9/2005

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Determining Event Energies

• The “visible” energy is calculated from the amount of photo-electrons correcting for spatial detector response. • The “real” energy is calculated from the visible energy correcting for Cherenkov photons and scintillation light quenching. 8/9/2005

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Tracking Muons

Monte Carlo (line) and Data (+)

Detecting Antineutrinos with KamLAND Delayed • KamLAND (Kamioka Liquid scintillator AntiNeutrino Detector) • Inverse beta decay e + p → e+ + n E ~ Te + 1.8MeV

Prompt

0.5 MeV  e 0.5 MeV 

2.2 MeV e+ n p

p

d

e

• The positron loses its energy then annihilates with an electron. • The neutron first thermalizes then captures a proton with a mean capture time of ~200ms. 8/9/2005

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

Selecting Geoneutrino Events Delayed Prompt

0.5 MeV 0.5 MeV



2.2 MeV

 e+



• • • • • • •

Δr < 1m 0.5μs < ΔT < 500μs 1.7MeV < E,p< 3.4MeV 1.8MeV < Ed< 2.6MeV Veto after muons Rp, Rd < 5m ρd>1.2m

*These cuts are different from the reactor antineutrino event selection cuts because of the excess background events for lower geoneutrino energies. 8/9/2005

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Outline • • • •

Geoneutrinos KamLAND Background Events Results

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Reactor Background Introduction Geoneutrinos

KamLAND

Reactor Background with oscillation

• KamLAND was designed to measure reactor antineutrinos. • Reactor antineutrinos are the most significant background. 8/9/2005

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Reactor Background Measurement • Reactor antineutrino signals are identical to geoneutrinos except for the prompt energy spectrum. • To calculate the reactor antineutrino interaction rate per target proton per year, we need to know the neutrino oscillation parameters, the detection cross-section (~0.2%) and each reactor’s • • • •

Location Reactor thermal power (~2.1%) Fuel composition (~1.0%) Antineutrino spectrum (~2.5%)

Long-lived Reactor Background Fractional Increase in energy spectra 235U

fission products

239Pu

fission products

Antineutrino Energy[MeV] Kopeikin et al. Physics of Atomic Nuclei 64 (2001) 849

• Fission fragments with half-lives greater than a few hours (97Zr, 132I, 93Y, 106Ru, 144Ce, 90Sr) may not have reached equilibrium. • The reactor antineutrino spectrum is based on the measured β spectrum after ~1day exposure of 235U, 239Pu, and 241Pu to a thermal n flux. • Long-lived isotopes occur in the core and spent fuel. • Spent fuel is assumed to be at the reactor location.

13C(α,n)16O

Background

np scattering 13C(a,n)16O*

n(12C,12C*)n

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• Alpha source, 210Po→206Pb+α. • Natural abundance of 13C is 1.1% • 13C(α,n)16O. • n loses energy creating a prompt event, and is later captured creating a delayed event.

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Cosmic Muon Induced Background Muon Veto

Fiducial Volume

• Muons produce unstable isotopes and neutrons as they go through the detector. • 9Li and 8He -decay producing n, mimicking inverse -decay signals. • Any events after muons are vetoed. – 2ms after all muons – 2s within 3m cylinder of the muon track – 2s whole detector for muons with high light yield

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Random Coincidence Background • There is a probability that two uncorrelated events pass the coincidence cuts. • The random coincidence background event rates are calculated by different delayed event time window (10ms to 20s instead).

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Background Event Summary • The following is a summary of the expected numbers of background coincidence events. Background Source Reactor Short-lived isotope Long-lived Isotope LS Radioactivity (a,n) Random Coincidence Spontaneous Fissions Cascade Decays (,n) Muon Induced Spallation Products Fast Neutrons Neutrons Total 8/9/2005

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Events 80.4 1.91 42 2.38 < 0.1 negligible negligible 0.30 < 0.1 negligible 127.0

Error 7.2 0.24 11 0.01

0.05

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Pulse Shape Discrimination From AmBe source

Neutrons Gammas

PMT hits

S

 i

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n(ti )  e(ti ) ti n(ti )  e(ti )

• Antineutrino prompt event is caused by e+ whereas 13C(α,n)16O prompt event is caused by n. • These different prompt events produce different scintillation light time distributions allowing a statistical discrimination. SLAC seminar

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Pulse Shape Discrimination Part 2 From AmBe source

Neutrons

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Gammas

• This study assumes similarities in time distributions of positrons and gammas. • This method yields consistent 13C(α,n)16O background event rate.

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Outline • • • •

Geoneutrinos KamLAND Background Events Results

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Data-set • • • • •

From March, 2002 to October, 2004. 749.1±0.5 day of total live-time. (3.46 ± 0.17) × 1031 target protons. (7.09 ± 0.35) × 1031 target proton years. 0.687±0.007 of the total efficiency for geoneutrino detection. • 14.8 ± 0.7 238U geoneutrinos and 3.9 ± 0.2 232Th geoneutrinos are expected. 8/9/2005

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Geoneutrino Candidate Energy Distribution Expected total Candidate Data Expected total background

Expected reactor

(a,n) Random

Expected Th

Expected U

Rate Analysis • • • •

152 candidate events 127±13 expected background events. +19 25-18 geoneutrinos. +3.9 -31 5.1-3.6  10 e / (target proton-year) detected geoneutrino rate.

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Likelihood Analysis • Uses un-binned likelihood analysis. • Uses the expected prompt event energy distribution. • Uses the neutrino oscillation parameters determined from results of KamLAND reactor antineutrino and solar neutrino experiments.

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Log Likelihood Equation logL  

  2 N    NN

2 N

  N

dN E dE

2

U

  N

dPU E dE

  

 





2  1 dN Ei   2 m12 ,sin2 212 pa  1 qa  1   log     2 2 dE 2 N 2  2  i 1    p q N

Th

2

  dN E ;m

dPTh E dE

r



2 12

,sin2 212

dE

2

 p

a



dNa E / qa dE



other BG

 k

  

dNk E dE

•

N : Number of candidate events observed • N : Number of candidate events expected •

qa : (a,n) background energy scaling factor

•

pa : (a,n) background rate scaling factor

For given NU and NTh, log L is maximized by varying the other parameters.

How Many Geoneutrinos Did We See? Expected ratio from chondritic meteorites

Best fit 3 U geoneutrinos 18 Th geoneutrinos

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Expected result from reference Earth model

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How Many Geoneutrinos Did We See, Part 2? 2 = 2(logLmax - logL)

Expected result from reference Earth model

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Central Value 28SLAC seminar

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Reality Check… • Could all “geoneutrinos” come from an undiscovered uranium deposit? • Not likely • The antineutrino flux from a 100kton uranium deposit (the world’s largest) located 1km away from KamLAND would be only 3% of expected geoneutrino flux. 8/9/2005

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Conclusions • This is the first experimental investigation of geoneutrinos. • This is the first chemical analysis of the mantle of the Earth. • We observed 4.5 to 54.2 geoneutrinos with 90% C.L. • Scaling concentrations in all regions of our reference Earth model, the 99% upper limit on geoneutrino rate corresponds to radiogenic power from U and Th decays of less than 60TW. • The measurement is consistent with the current geological models.

Future of Geoneutrino Measurement with KamLAND • The reactor background is irreducible for KamLAND. • We are working on purifying the liquid scintillator, which will reduce the (a,n) background events. • More accurate (a,n) cross section can lower the error on the (a,n) background rate. – S. Harissopulos et al. submitted to Phys. Rev. C calculated new (a,n) cross sections with more accuracy. – G. Fiorentini et al. arXiv:hep-ph/0508048 recalculated the number of geoneutrinos using the above cross sections and our data. They claim that we detected 31+14 geoneutrinos, -13 ~2.5 above 0.

Future Geoneutrino Experiment Considerations • Location and geoneutrino data purity: – – – – –

No nearby nuclear reactors On oceanic crust to probe mantle On continental crust to probe continental crust Needs to be shielded from cosmic muons Low radioactive background

• People are talking about – Hawaii (oceanic crust with no reactors) – Canada, South Dakota, Australia, the Netherlands, and South Africa (continental crust with no reactors)

• Geoneutrino Meeting in Hawaii, December 2005 8/9/2005

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Acknowledgement • Prof. E. Ohtani (Tohoku University) and Prof. N. Sleep (Stanford University) • Japanese Ministry of Education, Culture, Sports, Science, and Technology • United States Department of Energy • Electric associations in Japan: Hokkaido, Tohoku, Hokuriku, Chubu, Kansai, Chugoku, Shikoku, and Kyushu Electric Companies, Japan Atomic Power Co. and Japan Nuclear cycle Development Institute • Kamioka Mining and Smelting Company 8/9/2005

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KamLAND Collaborators T. Arak i1, S. Enomoto 1, K. Furuno1, Y.Gando1, K. Ichim ura 1, H. Ikeda 1, K. Inoue1, Y. Kishim oto1, M. Koga 1, Y. Koseki1, T. Maeda 1, T. Mitsui1, M. Motoki1, K. Nakajim a1, H. Ogawa 1, M. Ogawa 1, K. Owada1, J.-S. Ricol1, I. Shimi zu 1, J. Shirai1, F. Suekane1, A. Suzuk i1, K. Tada 1, S.Takeuc hi1, K. Tamae1, Y. Tsuda 1, H. Watanabe1, J. Busenitz2, T. Classen2, Z. Djurcic2, G. Keefer2, D. Leonard2, A. Piepke2, E. Yakushev2, B.E. Berger 3, Y.D. Chan3, M.P. Decowski3, D.A. Dwyer3, S.J. Freedman3, B.K. Fujikawa 3, J. Goldman3, F. Gray3, K.M. Heeger 3, L. Hsu 3, K.T. Lesko3, K.-B. Luk 3, H. Murayama3, T. OΥDonnell 3, A.W.P. Poon3, H.M. Steiner3, L.A. Winslow3, C. Mauger 4, R.D. McKeown4, P. Vogel4, C.E. Lane5, T. Mil etic5, G. Guilli an6, J.G. Learned6, J. Maricic6, S. Matsuno6, S. Pakvasa6, G.A. Horton-Smith7, S. Dazeley8, S. Hatakeyama8, A.Rojas8, R. Svoboda8, B.D. Dieterle9, J. Detwil er10, G. Gratta10, K. Ishii 10, N. Tolich10, Y. Uchida10, M. Batygov11, W. Bugg11, Y. Efremenko11, Y. Kamyshkov11, A. Kozlov11, Y. Nakamura 11, H.J. Karwowski12, D.M. Markoff12, K. Nakamura 12, R.M. Rohm12, W. Tornow12, R. Wendell 12, M.-J. Chen13, Y.-F. Wang13, and F. Piquemal14 1

Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan

2

Department of Physics and Astronomy, University of Alabama, Tuscaloosa, Alabama 35487, USA

3

Physics Department, University of California at Berkeley and Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

4

W. K. Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, California 91125, USA

5

Physics Department, Drexel University, Philadelphia, Pennsylvania 19104, USA

6

Department of Physics and Astronomy, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA

7

Department of Physics, Kansas State University, Manhattan, Kansas 66506, USA

8

Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803, USA

9

Physics Department, University of New Mexico, Albuquerque, New Mexico 87131, USA

10

Physics Department, Stanford University, Stanford, California 94305, USA

11

Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA

12

Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA and Physics Departments at Duke University, North Carolina State University, and the University of North Carolina at Chapel Hill 13

Institute of High Energy Physics, Beijing 100039, People's Republic of China

14

CEN Bordeaux-Gradignan, IN2P3-CNRS and University Bordeaux I, F-33175 Gradignan Cedex, France

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Geoneutrino Results in Nature

Nature 436, 499-503 (28 July 2005) | doi: 10.1038/nature03980 http://www.nature.com/nature/journal/v436/n7050/full/nature03980.html 8/9/2005

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